Thermoelectric materials are a class of materials that efficiently convert thermal energy to electrical energy (Seebeck effect) and vice versa (Peltier effect). The “Seebeck effect” is the phenomenon underlying the conversion of heat energy into electrical power and is used in thermoelectric power generation. The “Peltier effect” is the opposite the Seebeck effect and is phenomenon in which heat absorption accompanies the passage of current through the junction of two dissimilar materials. The Peltier effect is used in thermoelectric refrigeration and other cooling applications. In addition, thermoelectric materials are used in heating applications and thermoelectric sensing devices.
Only certain materials have been found usable for the Seebeck and Peltier effect to be observed. Some thermoelectric materials are semiconducting or semi-metallic. Such materials conduct electricity by using two types of carriers: electrons and holes. When one atom in a crystal is replaced by another atom with more valence electrons, the extra electrons from the substituting atom are not needed for bonding and can move around throughout the crystal. A semiconductor is called n-type if the conducting carriers are electrons. On the other hand, if an atom in the crystal is replaced with an another different atom having fewer valence electrons, one or more bonds are left vacant and thus positively charged “holes” are produced. A semiconductor is called p-type if the conducting carriers are holes. In the above-mentioned thermoelectric devices, both n-type and p-type thermoelectric materials are typically needed. Samples of n-type and p-type semiconductor materials used in thermoelectric devices are often referred to as “n-legs” and “p-legs”.
Devices made from thermoelectric materials are environmentally benign power sources that may provide a solution to today's energy problems. These devices convert thermal energy directly into electrical energy, can utilize waste heat, require minimal maintenance and can be segmented to operate over a large temperature range (300-1275 K), thus, they carry the potential of assisting the efforts to maintain and protect the environment. An electric power generator based on thermoelectric materials does not use moving parts like conventional power generators. This feature significantly enhances the reliability of the thermoelectric devices by avoiding mechanical wear of moving parts and corresponding failure. Thermoelectric devices may aid in the elimination of chlorofluorocarbons, which are used in most compressor-based refrigerators, as well as the conversion of waste heat into beneficial electrical power, for example, containing the heat produced from an automobile's engine or exhaust system and converting it into auxiliary power. Such devices allow operations in hostile environments such as in high temperature conditions (e.g., 1173 K) without human attendance. In addition, these devices have the potential of providing a power source that may outperform batteries. Overall, thermoelectric modules may be an asset for countless applications, many that would be of interest to automotive companies, appliance manufacturers, NASA, and the armed forces.
To determine the thermoelectric efficiency of a thermoelectric device, there are two primary parameters that govern performance; the temperature difference (ΔT=Th−Tc) across the module and the thermoelectric figure of merit (zT) of the materials. The temperature difference between the hot (Th) and cold (Tc) sides of a thermoelectric device sets the upper limits of efficiency through the Carnot efficiency, ηc=ΔT/Th (G. J. Snyder, Applied Physics Letters 84:2436-2438, 2004). The materials segmented in the n- and p-legs of the device determine how close the efficiency can be to the Carnot maximum through zT. Here, zT=α2T/ρκT (α: Seebeck coefficient (μV/K), T: temperature (K), ρ: electrical resistivity (mOhms cm), κT: thermal conductivity (mW/cm-K)) (F. J. DiSalvo, Science, Washington, D.C. 285:703-706, 1999). In addition, when segmenting materials for high efficiency, large temperature difference applications, the thermoelectric compatibility factors (s=[(1+zT)1/2−1]/αT) of the materials need to be similar (G. J. Snyder, Applied Physics Letters 84:2436-2438, 2004).
Good thermoelectric compounds are those that result in low electrical resistivity and thermal conductivity values and large Seebeck coefficient values (F. J. DiSalvo, Science, Washington, D.C. 285:703-706, 1999). It has been observed that typically small band-gap, semiconducting materials with carrier concentrations within the 1019-1021 cm−3 range work better than metallic or insulating materials (G. Mahan et al., Physics Today 50:42-47, 1997). In addition, a large unit cell, heavy atoms, and structural complexity are also predicted to result in low thermal conductivity and therefore high thermoelectric efficiency.
State of the art thermoelectric devices are typically based on Bi2Te3—Sb2Te3 alloys for room temperature applications and PbTe or GeTe based compounds for power generation applications up to about 500° C. Silicides such as FeSi2 and SiGe, which are current state of the art thermoelectric conversion materials for high temperatures (above 873 K), have a low figure of merit (zT) (FeSi2=0.2 or less; p-SiGe zT=0.6) (C. Wood, Energy Conversion and Management 24:331-43, 1984; 0. Yamashita and N. Sadatomi, Journal of Applied Physics 88:245-251, 2000).
Other materials that may be useful for thermoelectric applications include intermetallic clathrates, complex chalcogenides, half-Heusler alloys and antimonide skutterudites (N. L. Okamoto et al., Materials Research Society Symposium Proceedings 793:187-192, 2004; B. C. Sales et al., Science, Washington, D.C. 272:1325-1328, 1996; S. W. Kim et al., Science and Technology of Advanced Materials 5:485-489, 2004; D. Bilc et al., Physical Review Letters 93:146403/1-146403/4, 2004), as well as super-lattice thin film structures such as Bi2Te3/Sb2Te3 and PbSe0.98Te0.02/PbTe (R. Venkatasubramanian, E. Siivola et al., Nature 413:597-602, 2001; T. C. Harman et al., Journal of Electronic Materials 25: 1121-1127, 1996; H. Beyer et al., Applied Physics Letters 80, 1216-1218, 2002). However, out of the numerous compounds achieving breakthrough figures of merit, all these are in the room temperature to moderate temperature range of 300-900 K. Moreover, many thermoelectric applications require large quantities of material, making thin film systems an unrealistic option and bulk samples more desirable.
Based on the potential applications of thermoelectric materials, as well as the limitations of the current art, there is a significant need in the art for more efficient thermoelectric compounds and devices.